Introduction to Fuel Cells and Hydrogen Technology
Content
Introduction to fuel
by Brian Cook
Whereas the 19th century was the century of the steam engine and the 20th century was the century ofthe internal combustion engine, it is likely that the 21st century will be the century Ofthefuel cell. Full cells are now on the verge of being introduced commercially, revolutionising the way we presently produce power. Fuel cells can use hydrogen as a fuel, ofering the prospect of supplying the world with clean, sustainable electrical power..
A
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fuel cell by definition is an electrical cell, w h c h unhke storage cells can be continuously fed with a fuel so that the electrical power output is sustained indefinitely.’ It converts hydrogen, or hydrogencontaining fbels, &rectly into electrical energy plus heat through the electrochemical reaction of hydrogen and oxygen into water. The process is that of electrolysis in reverse: overall reaction: 2H2 (gas) + 0 2 (gas)+2H20 +energy
Because hydrogen and oxygen gases are electrochemically converted into water, fuel cells have many advantages over heat engines. These include: h g h efficiency, virtually silent operation and, if hydrogen is the fuel, there are no pollutant emissions. If the hydrogen is produced from renewable energy sources, then the electrical power produced can be truly sustainable. The two principal reactions in the burning of any hydrocarbon he1 are the formation of water and carbon &oxide. As the hydrogen content in a fuel increases, the formation ofwater becomes more significant, resulting in proportionally lower emissions of carbon dioxide (Fig. 1).As fuel use has developed through time, the percentage of hydrogen content in the fuels has increased. It seems a natural progression that the fuel of the future will be 100%hydrogen.
in 1839. The principle was dxovered by accident during an electrolysis experiment. When Sir W&am disconnected the battery from the electrolyser and connected the two electrodes together, he observed a current flowing in the opposite drection, consuming the gases of hydrogen and oxygen (Fig. 2). He called this device a ‘gas battery’. His gas battery consisted of platinum electrodes placed in test tubes of hydrogen and oxygen, immersed in a bath of ddute sulphuric acid. It generated voltages of about 1V. In 1842 Grove connected a number of gas batteries together in series to form a ‘gas chain’. He used the electricity produced from the gas chain to power an electrolyser, splitting water into hydrogen and oxygen (Fig. 3). However, due to problems of corrosion of the electrodes and instabdity of the materials, Grove’s fuel cell was not
History o fuel cells f
The kas battery’ Sir W a a m Grove (1811-96), a British l a y e r and amateur scientist, developed the first fuel cell
Fig. 1 Trends in the use of fuels. As fuel has developed through time, the percentage of hydrogen content in the fuel has increased
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Fig. 2 Principle of an electrolyser (left) and a fuel cell (see Reference 2)
practical. As a result, there was little research and further development oL@el cells for many years to follow.
T h e 'Baconfuel cell' Significant work on fuel cells began again in the 1930s, by Francis Bacon, a chemical engineer at the University of Cambridge, UK. In the 1950s Bacon successfully produced the first practical fuel cell, which was an &&ne version (Fig. 4). It used an alkaline electrolyte (molten KOH) instead of d h t e sulphuric
acid. The electrodes were constructed of porous sintered nickel powder so that the gases could di&e through the electrodes to be in contact with the aqueous electrolyte on the other side of the electrode. Ths greatly increased the contact area contact between the electrodes, the gases and the electrolyte, thus increasing the power density of the fuel cell. In addition, nickel was much less expensive than platinum. The chemical reactions in the alkaline fuel cell are:
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Fig. 3 Grove's 'gas battery' (1839)produced a voltage of about 1V (left); Grove's 'gas chain' powering an electrolyser (1842) (see Reference 3)
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Fig. 4 Bacon's laboratory at the Department of Chemical Engineering, University of Cambridge (1955). A fuel cell can be seen being assembledon the left of the picture (photo: courtesy of Department of Chemical Engineering, University of Cambridge)
anode reaction: 2H2 + 4OH- + 4&O+ 4ecathode reaction: 0 2 + 4e- + 2H20 + 4 0 H overall reaction: 2Hz + 0 2 -+2 H20
(2) (3) (4)
Fuel cells-for NASA For space applications, fuel cells have the advantage over conventional batteries in that they produce several tinies as much energy per equivalent unit ofweight. In the 1960s, International Fuel Cells in Windsor, Connecticut, USA, developed a fuel cell power plant Alkaline f u e l cellsfor terrestrial applications Compared with other types of fuel cells, the alkaline for the Apollo spacecraft. The plant, located in the variety offered the advantage of a high power to weight service module of the spacecraft, provided both electricity as well as drinking water for the astronauts on their journey to the moon. It could supply 1.5kW of continuous electrical power. Fuel cell performance during the Apollo missions was exemplary. Over 10 000 hours of operation were accumulated in 18 nlissions, without a single in-flight incident (Internet source: IFC). In the 1970s, International Fuel Cells developed a more powerfii alkaline fuel cell for NASA's Space Shuttle Orbiter (Fig. 5). The Orbiter uses three fuel cell power plants to supply all the electrical needs during bght. There are no backup batteries on the Space Shuttle, and as such, the fuel cell power plants nust be highly reliable. The power plants are fuelled by hydrogen and oxygen from cryogenic tanks and provide both electricalpower and drinking water. Fig. 5 NASA Space Shuttle Orbiter fuel cell, one of three fuel cells aboard the Each fuel cell is capable of supplying Space Shuttle. These fuel cells provide all the electricity as well as drinking water when the Space Shuttle is in flight. It produces 12kW(e) and occupies 154 litres 12kW continuously, and up to (photo: courtesyof NASA)
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16kW for short periods. The Orbiter ukts represent a significant technology advance over Apollo, producing about ten times the power from a sindar sized package. In the Shuttle progranme, the fuel cells have demonstrated outstanding reliabhty (over 99% availabhty). To date, they have flown on 106 nlissions and clocked up over 82000 hours of operation (Internet source: NASA).
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Fig. 6 Two prototype automobiles powered bv Ballard fuel cells, the NECAR 5 and JEEP Commander, from DaimlerChrysler (photo: courtesy of DaimlerChrysler)
ratio. This was primarily due to intrinsically faster lanetics for oxygen reduction to the hydroxyl anion in an alkahne environment. Therefore alkaline fuel cells were ideal for space applications. However, for terrestrial use, the primary dmdvantage of these cells is that of carbon dioxide poisoning of the electrolyte. Carbon &oxide is not only present in the air but also present in reformate gas, the hydrogen rich gas produced from the reformation of hydrocarbon fuels. In the poisoning of an &&ne fuel cell, the carbon dioxide reacts with the hydroxide ion in the electrolyte
to form a carbonate, thereby reducing the hydroxide
ion concentration in the electrolyte. This reduces the overall efficiency of the fuel cell. The chemical equation for carbon &oxide reacting with a potassium hydroxide electrolyte is:
Because of the complexity of isolating carbon dioxide fiom the &&ne electrolyte in fuel cells for terrestrial applications, most &el cell developers have focused their attention on developing new types using electrolytes that are non-alkaline. These fuel cells include: solid oxide fuel cells (SOFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC) and polymer electrolyte membrane or proton exchange membrane (PEM) fuel cells. PEM fuel cell In the early 1960s, General Electric (GE) also made a significant breakthrough in fuel cell technology. Through the work of Thomas Grubb and Leonard Niedrach, the company invented and developed the first polymer electrolyte membrane (PEM) fuel cell. It was initially developed under a programme with the
US Navy’s Bureau of Ships and US Army Signal Corps to supply portable power for personnel in the field. Attracted by the possibility of using GE’s PEM fuel cell on the Apollo missions, NASA tested its potential to provide auxiliary power onboard its Gemini spacecrafi. The Gemini space programme consisted of 12 flights in preparation for the Apollo missions to the moon. For lunar fights, a longer power source was required than could be provided by batteries, which had been used on previous space flights. Unfortunately, the GE fuel cell, model PB2, encountered technical difficulties prior to launch, including the leakage of oxygen through the membrane. As a result the Gemini missions 1 to 4 flew on batteries instead. The GE fuel cell was redesigned and a new model, the P3, successhlly operated on the Gemini fights 6 to 12. The Gemini fuel cell power plant consisted oftwo &el cell battery sections, each capable of producing a maximum power of about 1000W (Internet source: NASA). A further limitation of the GE PEM fuel cell at that time was the large quantity of platinum required as a catalyst on the electrodes. The cost of PEM fuel cells was prohbitively high, restricting its use to space applications. In 1979, Geoffrey Ballard, a Canadian geophysicist, chemist Keith Prater and engineer Paul Howard established the company Ballard Power Systems. In the early 1980s, Ballard took the abandoned GE fuel cell, whose patents had expired and searched for ways to improve its power and build it out of cheaper
material^.^
Worlung on a contract fiom the Canadian Department of National Defence, Ballard developed fuel cells with a significant increase in power density while reducing the amount of platinum required. From these developments it was recognised that fuel cells
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could be made smaller, more powerful and che?ply enough to eventually replace conventional power technologies. Ballard Power Systems has since grown to become recognised as a world leader in PEM fuel cell technology, developing fuel cells with power outputs ranging from 1kW, for portable and residential applications, through to 250 kW for distributrd power. Ballad has formed alliances with a wide range of companirs, including DainderChrysler,Ford, EBARA in Japan and ALSTOM in France. In the late 1980s and early 1990s Los Alamos National Laboratory and Texas A&M Uuiversity also made significant developments to the PEM fuel cell. They also found ways to signiticantly reduce the amount ofplatinum required and developed a niethod to h i t catalyst poisoning due to the presence of trace impurities in the hydrogen fuel (Internet source: Los Alamos National Laboratory).
Fuel cell applications
Panspurtation
The Cahfornia Low Emission Vehicle Program, administered by the California Air Resources Board (CARB), has been a large incentive for automobile manufacturers to actively pursue fuel cell development. This programme requires that, beginning in 2003, 2% of passenger cars delivered for sale in California from medium or large-sized manufacturers must be zero emission vehicles, called ZEVs. Either automobiles powered by batteries or those powered by fuel cells inert these requirements, as thc only output of a hydrogen fuel cell is pure water. The N E C A R 5 (Fig. 6) is the latest prototype fuel cell automobile by DaimlerChrysler. This automobile is fuelled with liquid methanol which is converted into hydrogen and carbon dioxide through use of an onboard fuel processor. The vehicle has virtually no pollutant emissions of sulphur dioxide, oxides of nitrogen, carbon nionoxide or particulates, the primary pollutants of the internal combustion engine. The eficiency of a fuel cell engine is about a factor of two higher than that of an internal combustion engine and the output of carbon dioxide, when fuelled with a hydrocarbon fuel, is considerably lower. The NECAR 5 drives and feels like a 'normal; car. It has a top speed of over 1 5 0 h d h (90mph), with a power output of75 kW (100bp). It is also believed that this vehicle wlll require less maintenance. It combines the low emission levels, the quietness and the smoothness associated with electric vehicles, while delivering a performance sinular to that ofan automobile with an internal combustion enginr. In April I Y99 the California Fuel Cell Partnership was developed. Foundmg members included DainderChrysler.the California An Resources Board, the Cahfornia Energy Commission, Ballard Power Systems, Ford, Shell and Texaco. The primary goals of the partnership are to:
Fig. 7 PEM fuel-cell distributed power plant. This unit, produced by Ballard Power Systems, provides 250kW heat and electricity which is enough power for a small building, a school or a community of up to 50 homes (photo: courtesy of Ballard Power Systems)
Demonstrate vehiclr technology by operating and testing the vehicles under real-world conditions in California. Demonstrate the viability of alternative fuel infrastructure technology, including hydrogen and methanol stations. Explorr the path to conunercialisation, from identifylng potential problcms to developing solutions. Increase public awareness and enhance opinion about fuel cell electric vehicles, preparing the market for commercialisation.
Fig. 8 Fuel-cell cogeneration power plant for residential applications, providing 7kW heat and electricity, enough power for a modern energy efficient home (photo: courtesy of Plug Power)
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Fig. 9 Prototype portable fuel cell providing 50W electrical power, produced by Heliocentris. The fuel cell is contained in the upper compartment; the hydrogen is stored in a metal hydride canister in the lower compartment
Since then new participants include General Motors, Honda, Hyundai, Nissan, Toyota, Volkswagen, British Petroleum, Exxon Mobil, US Department of Energy and US Department ofTransportation. To date five of the world's leading auto manufacturers have announced that they plan to introduce fuel cell automobiles beginning in the 2003 to 2005 timeframe. There are also plans {or buses and truck, all powered with fuel cell engines. In 2000, Ballard completed a two-year program of in-service field testing with six fuel cell buses, thrrc in Vancouver, British Colunibia, and three in Chicago. Objectives of the 6eld test included gathering data on performance and maintenance for me in Ballard's future heavy-duty engine designs, assessing public acceptance, and determining the needs of transit authorities and users of the buses. The results of the tests were exeniplary-the six buses travelled alnlost 75 000 miles and carried over 200 000 passengen. Thirty new transit buses powered by Ballard's heavy-duty fuel cell engines will be introduced to ten European cities beginning in 2003 for additional demonstration. The resulting data will be used to further develop a commercial fuel cell bus. Dbtribured power p x e r d t i o n Electrical energy demands throughout the world are continuing to increase. In Canada the demand is growing at an annual rate of approximately 2.6%. In America the rate is about 2.4%?, and in developing countries it is appmximatcly 696." How can these energy demands be niet responsibly and safely? Distributed power plants using fuel cells can provide part of the solution.
Distributed or 'decentrahsed' power plants, coutrastrd with centmlised power plants, are plants located close to the consumer, with the capability of providing both heat and electrical power (a combination known as 'cogeueration'). Heat, the byproduct ofelectrical power generation, is transferred from the fuel cell to a heat exchanger. The exchanger transfers the heat to a water supply, providing hot watcr to local customers. The overall efficiency of a cogeneration system can he iu excess of SOY,, comparatively high compared to a system producing electricity alone. An increase in efficiency naturally corresponds to a decrease in fuel consuniption. Distributed power plants have many additional advantages. For example, they can provide power to a remote location without the need of transporting electricity through transmission lines from a central plant. There is also an efficiency benefit in that the cost of transporting fuel is more than ofiet by the elinunation of the electrical losses of transmission. The ability to quickly build up a power infrastructure in developing nations is ohen cited. Using fuel cell power plants obviates the need for an electrical grid. Distributed power plants can provide either primary or back-up power. In primary applications they can provide base-load power, operating virtually continuously from the consumption of natural gas, reducing the demand from the electrical grid. This not only decreases the cost ofdisplaced power, but can also result in a reduction of demand charges imposed by the utility Should the power plant provide an excess of electricity, the excess can be fed back into the electrical grid, resulting in additional savings. In case of a power outagc on the grid, a distributed power plant can continue to pmvidt! power to cssentid services; eliminating the need for both an uninterruptible power supply (UPS),presently handled by lead-acid battery banks, and a standby generator, for exteiided period? of power outage. An additional quality of a fuel cell power plant for UPS applications is that the average 'down time' is anticipated to be low, 3.2 to 32 seconds per year against typically nine hours for a conventional battery-hank UPS (Internrt source: H D R Engineering). For industries where UPS systems are critical, such as banking, minimising dowii time is of utmost importance. Other applications for fuel cell dmributed power plants are also possible, e.g. stand-alone back-up power generators. The PEM fuel cell plant can be started in seconds, supplying power for as long as required from stored hydrogen, producing electrical power clearlly and virtually silently. Shown in Fig. 7 is a prototype l i d cell distributed power plant, by Ballard Power Systems. This unit pmvides 2SOkW of electricity and an cyuivalent amount ofheat. This is enough power for a community of about SO homes, or a small hospital or a remote school. This particular unit incorporates a fuel processor so that natural gas cau be used as a fiiel. The fuel processor convrro the natural gas, through the
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process of reformation, into a hydrogen-rich gas composed primarily of hydrogen and carbon dioxide. The hydrogen is used by the fuel cell and the carbon dioxide is released into the atmosphere. Eventually as an infrastructure for hydrogen develops, these units could be powered with hydrogen directly without the need for a fuel processor. Ballard Power is presently field-testing five of these units in the United States, Germany, Japan and Switzerland, with four more units planned for 2002. Testing is expected to continue until 2004.
compressed hydrogen
lithium-ionbattery
lead-acid battery
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Residential power Fuel cell power plants are also > being developed by several g 100 c manufacturers to provide 50 electricity and heat to single-fady homes. Fuelled by either natural 0 compressed hydrogen lithium-ionbattery lead-acidbattery gas or propane, these plants will be able to supply base-load power or storage density by weight all the electricity required by a modern-day home. Fig. 10 Comparison of the energy density of compressed hydrogen (3000 psi) Ballard Power Systems, in against lithium-ion and lead-acid batteries collaboration with its associate purchase and install a larger number of units in a variety company Ebara Ballard, partner Ebara Corporation, and co-developer Tokyo Gas, has developed a 1kW of locations in the future. fuel cell generator designed to supply both base-load Portable power electrical power as well as heat to a dwelling. This unit can also be fuelled by natural gas. It does not provide Several manufacturers are also developing fuel cell enough power to supply the total electrical demands of power supplies for portable applications, providmg a a residence, but it does shift a portion of the demand few watts up to several kilowatts of electricity (Fig. 9). from the electrical grid to natural gas. The electrical Fuelled by stored natural gas, propane, methanol or efficiency of this fuel cell system is rated at 42% and the hydrogen gas, portable fuel cells may one day replace heat efficiency is rated at 43%. Therefore the combined both gasoline and desel-engine generatorsfor portable applications as well as conventional batteries for uses cogeneration efficiency of the system can be as h g h as such as remote lighting, laptop computers and mobile 85%. This particular generator is targeted at the Japanese residential market. Ballard’s goal is to phones. commence sales of these units in 2004. Compared with engine-driven mobile electrical generators, fuel cells have the significant advantage of Plug Power, based in Latham New York has developed a new fuel cell power plant that supplies being quiet and having low emissions. As they have few 5 kW of electricity plus heat, using natural gas as a fuel moving parts (only external pumps and fans) they (Fig. 8). Dependmg on the size and location of the operate virtually silently. If stored hydrogen is the fuel, house this could be enough power to supply the again the only emission is pure water. electrical demand of a modern energy efficient home. A significant advantage of the he1 cell over its battery Initially these fuel cell power plants will be operated in counterpart is that of its energy density (Fig. 10). parallel with the grid. Eventually they will be able to Portable power packs using fuel cells can be lighter and operate either grid parallel or grid independent, smaller in volume for an equivalent amount of energy, possibly supplying the entire power for a modern particularly the direct methanol &el cell. Note that the home. Plug Power is presently installing and testing comparison here is the he1 tank. these units in selected sites throughout North America, Europe and Japan. In 2001, 75 units were delivered to ‘ T h ef u e l cell makes sense when the energy storage required Long Island Power Authority (LIPA) to supplement the by an application represents many hours o operation at full f grid and provide learning experience for LIPA power. T h e durability o batteries i n this sort o application f f employees. This experience will enable LIPA to is at best a f e w hours. T h e size, weight, and cost o energy f
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discharge. Also the charge capacity of a rechargeable battery decreases with the number of times of charge and discharge. Conversely, provided that the hydrogen supply is sealed correctly, a fuel cell will not discharge over time, maintaining its full charge capacity almost indefinitely. Direct methanol fuel cells were invented and initially developed at theJet Propulsion Laboratory in Pasadena, California. They were designed to supply electricity for field troops in the Armed Forces and for applications with NASA (Fig. 11). The direct methanol, fuel cell has the advantage over the hydrogen fuel cell in that they can use a liquid fuel, i.e. methanol without the need for external reforming. Liquid Fuel is easy to store and has a high energy density compared to compressed hydrogen. At present, the direct methanol fuel cell suffers From relatively low efficiency and high cost, owing to required platinum loadmg compared to that ofthe hydrogen fuel cell. However, as this improves, it is expected that the direct methanol fuel cell will play a leading role in providing power for portable and possibly transportation applications. Ballad Power Systems, Motorola, the Los AIanios National Laboratory and Manhattan Scientific are all actively pursuing the development of the direct methanol fuel cell. Motorola claim that a portable cell phone wiU he able to remain fully charged on standby for a month rather than &ys. The company has also announced that it plans to have its version commercially available in three to five years.
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Fig. 11 Prototype direct methanol fuel cell used as a lithium battely charger provides up to 20W electrical power (photo: courtesy of Jet Propulsion LaboratorieslNASA)
a fuel cell power plant easily out-competes batteries. You do have thefixed cost (and size and wekht) of the plant, which is afunction of power. 7his is why it is important to note that the advanrage offuel cells isfor low power, hkh eneryy applicationr.’ (Ric Pow of Pow Consulting, 2001)
storage f.r
Rechargeable batteries will discharge over time; the colder the ambient temperature the quicker they
The science of the PEM fuel cell
T h e chemistry o a single cell f In a PEM fuel cell, hvo half-cell reactions take place simultaneously,an oxidation reaction (loss ofelectrons) at the anode and a reduction reaction (gain ofelectrons) at the cathode. These two reactions make up the total oxidation-reduction (redox) reaction of the fuel cell, the formation of water from hydrogen and oxygen gases. As in an electrolyser, the anode and cathode are separated by an electrolyte, which allows ions to be transferred &om one side to the other (Fig. 12). The electmlyte in a PEM fuel cell is a solid acid supported within the membrane. The solid acid electrolyte is saturated with water so that thr transport of ions can proceed. The chemical reactions for a PEM fuel cell are:
anode reaction: &+2H+ + 2ecathode reaction: XO2 + 2e- + 2Hi+H?Og) overall reaction: H2 + X o + H 2 0 O2
(6)
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(8)
Fig. 12 Schematic of a single PEM fuel cell. When an electrical load is attached across the anode and the cathode of the fuel cell a redox reaction occurs. The working voltage produced by one cell in this process is between 0.5 and O W , dependingon the load. To create practicalworking voltages, individualfuel cells are stacked together in series to form a fuel cell stack
At the anode, the hydrogen molecules first come into contact with a platinum catalyst on the electrode surface. The hydrogen molecules break apart, bonding to the platinum sudace forming weak H-Pt bonds. As the hydrogen molecule is now broken the oxidation reaction can proceed. Each hydrogen atom releases its
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electron, which travels around the external circuit to the cathode (it is this flow of electrons that is referred to as electrical current). The remaining hydrogen proton bonds with a water molecule on the membrane surface, forming a hydronium ion (H@+). The hydronium ion travels through the membrane material to the cathode, leaving the platinum catalyst site free for the next hydrogen molecule. At the cathode, oxygen molecules come into contact with a platinum catalyst on the electrode surface. The oxygen molecules break apart bondmg to the platinum surface forming weak 0-Pt bonds, enabling the reduction reaction to proceed. Each oxygen atom then leaves the platinum catalyst site, combining with two electrons (which have travelled through the external circuit) and two protons (whch have travelled through the membrane) to form one molecule of water. The redox reaction has now been completed. The platinum catalyst on the cathode electrode is again f?ee for the next oxygen molecule to arrive. Ths exothermic reaction, the formation of water from hydrogen and oxygen gases, has an enthalpy of -286kJ of energy per mole of water formed. The &ee energy avadable to perform work decreases as a function of temperature. At 25"C, one atmosphere, the free energy avadable to perform work is about -237kJ/mole. Ths energy is observed as electricity and heat.
polytetrafluoroethylene (PTFE) chains
F F F F F F F F F F F F F F F
-C-C-C-C-C-C-C-C-C-C-c-c-c-c-c-
I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I F F F F F F F O F F F F F F F
Fig. 13 Chemicalstructure of a PEM fuel cell membranelong chains of PTFE (Teflon) with side chain ending with sulphonic acid (HSOs) (source: Reference 2)
Polymer electrolyte (or proton exchange) membrane ('EM) The membrane material used in a PEM cell is a polymer. PEMs are generally produced in large sheets. The electrode catalyst layer is applied to both sides, and is cut to the appropriate size. A single PEM sheet is typically 50-175pm thck, or around the thckness of 2-7 sheets of paper. A common PEM material used today is Nafion. Developed in the 1970s by Dupont, Nafion consists of polytetrafluoroethylene (PTFE) chains, commonly known as Teflon, forming the backbone of the membrane. Attached to the Teflon chains, are side chains endmg with sulphonic acid (HS03) groups (Fig. 13).A close-up view of the membrane material shows long, spaghetti-like chain molecules with clusters of sulphonate side chains (Fig. 14). An interesting feature of this material is that, whereas the long chain molecules are hydrophobic (repel water), the sulphonate side chains are hghly hydrophylic (attract water). For the membrane to conduct ions efficiently the sulphonate side chains must absorb large quantities of water. Withn these hydrated regions, the hydrogen ions of the sulphonic acid groups can then move heely, enabling the membrane to transfer hydrogen ions, in the form of hydronium ions from one side of the membrane to the other.
Cell voltage and ejiciiency If the hel, cell was perfect at transferring chemical energy into electrical energy, the ideal cell voltage
(thermodynamic reversible cell potential) of the hydrogen he1 cell would be at 25"C, one atmosphere, 1.23V. As the he1 cell heats up to operating temperature, around 80°C the ideal cell voltage drops to about 1.18V. However, there are many limiting factors that reduce the &el cell voltage further. The voltage out of the cell is a good measure of electrical
Fig. 14 Close-up of a PEM fuel cell membrane. The Figure shows long spaghetti-like chain moleculesof Teflon surrounding clusters of hydrated regions around the sulphonate side chains. The Teflon chains form the backbone of the membrane. The hydrated regions around the sulphonate side chains become the electrolyte (source: Reference 2)
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Fig. 15 Graph comparing carbon dioxide emissions of cars, using different types of fuel sources (reprinted by permission from Pembina Institute). a Car with internal combustion engine; b Fuel cell car with hydrogen produced from Alberta electric grid (coal generation); c Fuel cell car with onboard gasoline reformer; d Fuel cell car with onboard methanol reformer; e Fuel cell car with hydrogen produced from natural gas (distributed from urban retail outlets); f Fuel cell car with hydrogen produced from natural gas (made at large refineries)
300 250
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200 150 -
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efficiency; the lower the voltage, the lower the electrical efficiency and the more chemical energy is released in the formation of water and transferred into heat. The primary losses that contribute to a reduction in cell voltage are:
therefore important to remove this excess water, hence the term mass transport.
Direct methanol fuel cell
A direct methanol fuel cell also uses a PEM membrane. However, other catalysts in addtion to platinum are required on the anode side of the membrane to break the methanol bond in the reaction forming carbon dioxide, hydrogen ions and free electrons. As with the hydrogen fuel cell, the free electrons flow from the anode of the cell through an external circuit to the cathode and the hydrogen protons are transferred through the electrolyte nienibrane. At the cathode the Gee electrons and the hydrogen protons react with oxygen to form water. The chemical reactions of the direct methanol fuel cell are:
anode reaction: CH30H + H20 +COZ 6H' + 6e- (9) + cathode reaction: 31202 + 6Hf + 6e-+ 3H20 (10) overall reaction: CH30H + 3/202 -+ C02 + 2H20 (11)
Actiuation losses: Activation losses are a result of the energy required to initiate the reaction. This is a result of the catalyst. The better the catalyst the less activation energy is required. Platinum forms an excellent catalyst; however, there is much research underway for better materials. A limiting factor to power density available from a fuel cell is the speed at which the reactions can take place. The cathode reaction (the reduction of oxygen) is about 100 times slower than that of the reaction at the anode, thus it is the cathode reaction that limits power density. Fuel crossouer and internal currents: Fuel crossover and internal currents are a result of fuel that crosses directly through the electrolyte, from the anode to the cathode without releasing electrons through the external circuit, thereby decreasing the efficiency of the fuel cell. Ohmic losses: Ohmic losses are a result of the combined resistances of the various coniponents of the fuel cell. Ths includes the resistance of the electrode materials, the resistance of the electrolyte membrane and the resistance of the various interconnections. Concentration losses (also referred to as 'mass transport'): These losses result from the reduction of the concentration of hydrogen and oxygen gases at the electrode. For example, following the reaction new gases must be made immedately available at the catalyst sites. With the build up of water at the cathode, particularly at high currents, catalyst sites can become clogged, restricting oxygen access. It is
Where will the hydrogen come from?
One of the most important questions to be asked is: where will the hydrogen come from?A very interesting study published by the Pembina Institute, based in Calgary, Alberta, compared total carbon dioxide emissions of fuel cell vehicles using hydrogen produced from a variety ofmethods (Fig. 15). The results clearly show that the choice as to which method will be used to produce the hydrogen will be a critical environmental decision. For example, if hydrogen is produced from the electrolysis of water and the electrolysers are powered from the electrical grid, whereby the electricity is produced from a coal burning power station, then there will be no reduction in carbon dioxide emissions compared with the levels of the present day internal
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combustion engine. In fact, there will be an increase in metals and pollutants into the environment. If on the other hand the electrolyser is powered from a renewable energy source, through use of a solar panel, a wind turbine or a hydroelectric turbine, there will be no emissions of carbon dioxide. As the fuel cell industry grows there will likely be different approaches to niahng hydrogen in various areas of the world, depending on their local energy production methods.
solar cell
wind
micro hydro
Reformation of hydrocarbon fucls For the short term, because of the abundance of natural gas, the availability of Fig. 16 Electrical power from renewable energy sources. In the past, the limiting factors methanol and propane, and of renewable energy have been the storage and transport of that energy. With the use of the lack of a hydrogen an electrolyser, a method of storing and transporting hydrogengas, and a fuel cell, infrastructure, it is expected electricalpower from renewable energy sources can be deliveredwhere and when that hydrocarbon fuels will required, cleanly, efficiently and sustainably be the dominant fuels for stationary fuel cell applications. For as long as these These systems are truly sustainable, for as long as fuels are cheaply available, reformation of a there is sunlight there can be electrical power, available hydrocarbon fuel is the most cost efficient method of where and when required. producing hydrogen. In the reformation of a hydrocarbon fuel, however, there is an emission of Biological methods carbon &oxide. Although carbon dioxide is not Research and development is talung place on the considered a pollutant, controversy exists that manproduction of hydrogen from biological methods made emissions niay contribute to global warming. (biohydrogen). For example, Dr. A. Melis at the University of California, Berkeley, has discovered a Renewable energy systems metabolic switch in common green algae Hydrogen can be produced sustainably with no (chlamydomonas reinhardtill that causes the algae to emission of carbon &oxide from renewable energy oxidise water and to produce pure hydrogen gas when systems. An example of such a system is the use of a sulphur nutrients are depleted froin the growth solar panel, a wind turbine or a micro-hydro generator medium. This and other biohydrogen mechanisms are to convert the ra&ant energy of sunhght into electrical presently in the R&D stage but may one day provide power, which drives an electrolyser. The electrolyser the world with an additional source of hydrogen. breaks apart water producing hydrogen and oxygen gases. The hydrogen is stored for use by the fuel cell and Benefits and obstacles the oxygen is released into the atmosphere. Thus when the sun shines, the wind blows or the water flows, the Ben$ts electrolyser can produce hydrogen. Fuel cells are e$icient: They convert hydrogen and A power system incorporating hydrogen from oxygen directly into electricity, water and heat, with renewable sources and a fuel cell is a closed system, as no combustion in the process. The resulting none of the products or reactants, water, hydrogen and efficiency is between 50 and 80%, about double that oxygen, is lost to the outside environment. The water of an internal combustion engine. consumed by the electrolyser is converted to gases. The Fuel (ells are clean: If hydrogen i the fuel, there are no s gases are converted back to water. The electrical energy pollutant emissions &om a fuel cell itseE only the produced by the solar panel is transferred to cheinical n production of pure water. In contrast to a internal energy in the form ofgases. The gases can be stored and combustion engine, a bel cell produces no emissions of transported, to be reconverted back to electricity sulphur dioxide, which can lead to acid rain, nor nitrogen (Fig. 16). oxides whch produce smog nor dust particulates.
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Fuel cells are quiet: A b e l cell itself has no moving parts, although a fuel cell system may have pumps and fans. As a result, electrical power is produced relatively silently. Many hotels and resorts in quiet locations, for example, could replace dese1 engine generatorswith &el cells for both main power supply or for backup power in the event of power outages. Fuel cells are modular: That is, fuel cells of varying sizes can be stacked together to meet a required power demand. As mentioned earlier, fuel cell systems can provide power over a large range, &om a few watts to megawatts. Fuel cells are environmentally safe: They produce no hazardous waste products, and their only byproduct is water (or water and carbon dioxide in the case of methanol cells) and heat. Fuel cells may give us the opportunity to provide the world with sustainable electrical power. Obstacles At present there are many uncertainties to the success of fuel cells and the development of a hydrogen economy: Fuel cells must obtain mass-market acceptance to succeed: Thls acceptance depends largely on price, reliabhty, longevity of fuel cells and the accessibhty and cost of fuel. Compared to the price of present day alternatives e.g. desel-engine generators and batteries, fuel cells are comparatively expensive. In order to be competitive, fuel cells need to be massproduced and less expensive materials developed. An infvastructure for the mass-market availability of hydrogen, or methanolfuel initially, must also develop: At present there is no infi-astructure in place for either of these fuels. As it is we must rely on the
activities of the oil and gas companies to introduce
Conclusion
As our demand for electrical power grows, it becomes increasinglyurgent to find new ways ofmeeting it both responsibly and safely. In the past, the limiting factors of renewable energy have been the storage and transport of that energy. With the use of fuel cells and hydrogen technology, electrical power from renewable energy sources can be delivered where and when required, cleanly, efficiently and sustainably.
Acknowledgments
The author would &e to thank Rachel Browne for supplying the graphs and drawings and editing the text, and Ric Pow, h m Pow Consulting,Vancouver, Canada, for reviewing the paper and providmg t e c h c a l advice.
References
1 CONNIHAN, M. A,: ‘Dictionary of energy’ (Routledge and Kegan Paul, 1981) 2 LARMINIE, J., and DICKS, A,: ‘Fuel cell systems explained’ (John Wiley & Sons, 2000) 3 BERRY, M., and MACDONALD A.: ‘Energy through hydrogen’ Heliocentris 4 KOPPEL, T.: ‘Poweringthe future’(JohnWiley and Sons, 1999) 5 INTERNATIONAL ENERGY AGENCY ‘Energy policies of IEA countries’ (OECD Publications, 1997) 6 KHATIB,H.: ‘Electricalpower in developing countries’,Power EngineeringJournal, October 1998, 12, (lo), pp. 239-247 7 COLELL, H.: ‘Solar hydrogen technology’ (Heliocentris, 1998) 8 MELIS, A,, and HAPPE, T.: ‘Hydrogen production:green algae as a source of energy’, Plant Physiology, 2001, 127:pp. 740-748 9 THOMAS, S., and ZALBOWITZ, M.: ‘Fuel cells, green power’ &os Alamos Nanonal Laboratory, 1999, booklet)
internet sources
Power System: http://www.ballard.com/25Okstationary.asp HDR Engineering and Architecture: hap://www.hdrinc.com/ information/search.asp?PageID=476 IFC: http://www.in~rnationalfuelcells.com/spacedefe~e/ heritage. shtml Jet Propulsion Laboratory: http://www2.jpl.nasa.gov/til~/unages/ captiom/p486OO.txt Los Alamos National Laboratory: http://www.lanl.gov/worldview/ science/features/helcell. html NASA: Gemini: http://scienCe.ksc.nasa.gov/history/gemigemjnv/Fmini-v.htd Space Shuttle Orbiter: http://science.ksc.nasa.gov/shuttle/ technology/sts-newsref/sts-eps. html Pembina Institute: http://www.pembina.org/pubs/pa ibelcell.pdf Plug Power: http://www.plugpower.com/technology/ Smithsonian Institution: http://americanhistory.si.edu/csr/fuelcells/ pem/pemmain.hm
them. Unless motorists are able to obtain fuel conveniently and affordably, a mass market for motive applications will not develop. A t present a large portion of the investment infuel cells and hydrogen technology has come from auto manujicturers: However, if fuel cells prove unsuitable for automobiles, new sources of investment for &el cells and the hydrogen industry wdl be needed. Changes in government policy could also derail fuel cell and hydrogen technology development: At present stringent environmental laws and regulations, such as the Cahfornia Low Emission Vehcle Program, have been a great encouragement to these fields. Deregulation laws in the uthty industry have been a large impetus for the development of distributed stationary power generators. Should these laws change it could create
Ballard
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supplies to the world platinum market are &om South Africa, Russia and Canada. Shortages of platinum are not anticipated; however, changes in government policies could af€ect the supply.
cells and hydrogen’ technolib equipment for schools and universities. Mr Cook has a background in science kom University of British Columbia and electrical technology, and has a special interest in environmental sustainability. He can be contacted at: [email protected].
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by Brian Cook
Whereas the 19th century was the century of the steam engine and the 20th century was the century ofthe internal combustion engine, it is likely that the 21st century will be the century Ofthefuel cell. Full cells are now on the verge of being introduced commercially, revolutionising the way we presently produce power. Fuel cells can use hydrogen as a fuel, ofering the prospect of supplying the world with clean, sustainable electrical power..
A
(1)
fuel cell by definition is an electrical cell, w h c h unhke storage cells can be continuously fed with a fuel so that the electrical power output is sustained indefinitely.’ It converts hydrogen, or hydrogencontaining fbels, &rectly into electrical energy plus heat through the electrochemical reaction of hydrogen and oxygen into water. The process is that of electrolysis in reverse: overall reaction: 2H2 (gas) + 0 2 (gas)+2H20 +energy
Because hydrogen and oxygen gases are electrochemically converted into water, fuel cells have many advantages over heat engines. These include: h g h efficiency, virtually silent operation and, if hydrogen is the fuel, there are no pollutant emissions. If the hydrogen is produced from renewable energy sources, then the electrical power produced can be truly sustainable. The two principal reactions in the burning of any hydrocarbon he1 are the formation of water and carbon &oxide. As the hydrogen content in a fuel increases, the formation ofwater becomes more significant, resulting in proportionally lower emissions of carbon dioxide (Fig. 1).As fuel use has developed through time, the percentage of hydrogen content in the fuels has increased. It seems a natural progression that the fuel of the future will be 100%hydrogen.
in 1839. The principle was dxovered by accident during an electrolysis experiment. When Sir W&am disconnected the battery from the electrolyser and connected the two electrodes together, he observed a current flowing in the opposite drection, consuming the gases of hydrogen and oxygen (Fig. 2). He called this device a ‘gas battery’. His gas battery consisted of platinum electrodes placed in test tubes of hydrogen and oxygen, immersed in a bath of ddute sulphuric acid. It generated voltages of about 1V. In 1842 Grove connected a number of gas batteries together in series to form a ‘gas chain’. He used the electricity produced from the gas chain to power an electrolyser, splitting water into hydrogen and oxygen (Fig. 3). However, due to problems of corrosion of the electrodes and instabdity of the materials, Grove’s fuel cell was not
History o fuel cells f
The kas battery’ Sir W a a m Grove (1811-96), a British l a y e r and amateur scientist, developed the first fuel cell
Fig. 1 Trends in the use of fuels. As fuel has developed through time, the percentage of hydrogen content in the fuel has increased
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Fig. 2 Principle of an electrolyser (left) and a fuel cell (see Reference 2)
practical. As a result, there was little research and further development oL@el cells for many years to follow.
T h e 'Baconfuel cell' Significant work on fuel cells began again in the 1930s, by Francis Bacon, a chemical engineer at the University of Cambridge, UK. In the 1950s Bacon successfully produced the first practical fuel cell, which was an &&ne version (Fig. 4). It used an alkaline electrolyte (molten KOH) instead of d h t e sulphuric
acid. The electrodes were constructed of porous sintered nickel powder so that the gases could di&e through the electrodes to be in contact with the aqueous electrolyte on the other side of the electrode. Ths greatly increased the contact area contact between the electrodes, the gases and the electrolyte, thus increasing the power density of the fuel cell. In addition, nickel was much less expensive than platinum. The chemical reactions in the alkaline fuel cell are:
-12
Fig. 3 Grove's 'gas battery' (1839)produced a voltage of about 1V (left); Grove's 'gas chain' powering an electrolyser (1842) (see Reference 3)
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Fig. 4 Bacon's laboratory at the Department of Chemical Engineering, University of Cambridge (1955). A fuel cell can be seen being assembledon the left of the picture (photo: courtesy of Department of Chemical Engineering, University of Cambridge)
anode reaction: 2H2 + 4OH- + 4&O+ 4ecathode reaction: 0 2 + 4e- + 2H20 + 4 0 H overall reaction: 2Hz + 0 2 -+2 H20
(2) (3) (4)
Fuel cells-for NASA For space applications, fuel cells have the advantage over conventional batteries in that they produce several tinies as much energy per equivalent unit ofweight. In the 1960s, International Fuel Cells in Windsor, Connecticut, USA, developed a fuel cell power plant Alkaline f u e l cellsfor terrestrial applications Compared with other types of fuel cells, the alkaline for the Apollo spacecraft. The plant, located in the variety offered the advantage of a high power to weight service module of the spacecraft, provided both electricity as well as drinking water for the astronauts on their journey to the moon. It could supply 1.5kW of continuous electrical power. Fuel cell performance during the Apollo missions was exemplary. Over 10 000 hours of operation were accumulated in 18 nlissions, without a single in-flight incident (Internet source: IFC). In the 1970s, International Fuel Cells developed a more powerfii alkaline fuel cell for NASA's Space Shuttle Orbiter (Fig. 5). The Orbiter uses three fuel cell power plants to supply all the electrical needs during bght. There are no backup batteries on the Space Shuttle, and as such, the fuel cell power plants nust be highly reliable. The power plants are fuelled by hydrogen and oxygen from cryogenic tanks and provide both electricalpower and drinking water. Fig. 5 NASA Space Shuttle Orbiter fuel cell, one of three fuel cells aboard the Each fuel cell is capable of supplying Space Shuttle. These fuel cells provide all the electricity as well as drinking water when the Space Shuttle is in flight. It produces 12kW(e) and occupies 154 litres 12kW continuously, and up to (photo: courtesyof NASA)
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16kW for short periods. The Orbiter ukts represent a significant technology advance over Apollo, producing about ten times the power from a sindar sized package. In the Shuttle progranme, the fuel cells have demonstrated outstanding reliabhty (over 99% availabhty). To date, they have flown on 106 nlissions and clocked up over 82000 hours of operation (Internet source: NASA).
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Fig. 6 Two prototype automobiles powered bv Ballard fuel cells, the NECAR 5 and JEEP Commander, from DaimlerChrysler (photo: courtesy of DaimlerChrysler)
ratio. This was primarily due to intrinsically faster lanetics for oxygen reduction to the hydroxyl anion in an alkahne environment. Therefore alkaline fuel cells were ideal for space applications. However, for terrestrial use, the primary dmdvantage of these cells is that of carbon dioxide poisoning of the electrolyte. Carbon &oxide is not only present in the air but also present in reformate gas, the hydrogen rich gas produced from the reformation of hydrocarbon fuels. In the poisoning of an &&ne fuel cell, the carbon dioxide reacts with the hydroxide ion in the electrolyte
to form a carbonate, thereby reducing the hydroxide
ion concentration in the electrolyte. This reduces the overall efficiency of the fuel cell. The chemical equation for carbon &oxide reacting with a potassium hydroxide electrolyte is:
Because of the complexity of isolating carbon dioxide fiom the &&ne electrolyte in fuel cells for terrestrial applications, most &el cell developers have focused their attention on developing new types using electrolytes that are non-alkaline. These fuel cells include: solid oxide fuel cells (SOFC), phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC) and polymer electrolyte membrane or proton exchange membrane (PEM) fuel cells. PEM fuel cell In the early 1960s, General Electric (GE) also made a significant breakthrough in fuel cell technology. Through the work of Thomas Grubb and Leonard Niedrach, the company invented and developed the first polymer electrolyte membrane (PEM) fuel cell. It was initially developed under a programme with the
US Navy’s Bureau of Ships and US Army Signal Corps to supply portable power for personnel in the field. Attracted by the possibility of using GE’s PEM fuel cell on the Apollo missions, NASA tested its potential to provide auxiliary power onboard its Gemini spacecrafi. The Gemini space programme consisted of 12 flights in preparation for the Apollo missions to the moon. For lunar fights, a longer power source was required than could be provided by batteries, which had been used on previous space flights. Unfortunately, the GE fuel cell, model PB2, encountered technical difficulties prior to launch, including the leakage of oxygen through the membrane. As a result the Gemini missions 1 to 4 flew on batteries instead. The GE fuel cell was redesigned and a new model, the P3, successhlly operated on the Gemini fights 6 to 12. The Gemini fuel cell power plant consisted oftwo &el cell battery sections, each capable of producing a maximum power of about 1000W (Internet source: NASA). A further limitation of the GE PEM fuel cell at that time was the large quantity of platinum required as a catalyst on the electrodes. The cost of PEM fuel cells was prohbitively high, restricting its use to space applications. In 1979, Geoffrey Ballard, a Canadian geophysicist, chemist Keith Prater and engineer Paul Howard established the company Ballard Power Systems. In the early 1980s, Ballard took the abandoned GE fuel cell, whose patents had expired and searched for ways to improve its power and build it out of cheaper
material^.^
Worlung on a contract fiom the Canadian Department of National Defence, Ballard developed fuel cells with a significant increase in power density while reducing the amount of platinum required. From these developments it was recognised that fuel cells
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could be made smaller, more powerful and che?ply enough to eventually replace conventional power technologies. Ballard Power Systems has since grown to become recognised as a world leader in PEM fuel cell technology, developing fuel cells with power outputs ranging from 1kW, for portable and residential applications, through to 250 kW for distributrd power. Ballad has formed alliances with a wide range of companirs, including DainderChrysler,Ford, EBARA in Japan and ALSTOM in France. In the late 1980s and early 1990s Los Alamos National Laboratory and Texas A&M Uuiversity also made significant developments to the PEM fuel cell. They also found ways to signiticantly reduce the amount ofplatinum required and developed a niethod to h i t catalyst poisoning due to the presence of trace impurities in the hydrogen fuel (Internet source: Los Alamos National Laboratory).
Fuel cell applications
Panspurtation
The Cahfornia Low Emission Vehicle Program, administered by the California Air Resources Board (CARB), has been a large incentive for automobile manufacturers to actively pursue fuel cell development. This programme requires that, beginning in 2003, 2% of passenger cars delivered for sale in California from medium or large-sized manufacturers must be zero emission vehicles, called ZEVs. Either automobiles powered by batteries or those powered by fuel cells inert these requirements, as thc only output of a hydrogen fuel cell is pure water. The N E C A R 5 (Fig. 6) is the latest prototype fuel cell automobile by DaimlerChrysler. This automobile is fuelled with liquid methanol which is converted into hydrogen and carbon dioxide through use of an onboard fuel processor. The vehicle has virtually no pollutant emissions of sulphur dioxide, oxides of nitrogen, carbon nionoxide or particulates, the primary pollutants of the internal combustion engine. The eficiency of a fuel cell engine is about a factor of two higher than that of an internal combustion engine and the output of carbon dioxide, when fuelled with a hydrocarbon fuel, is considerably lower. The NECAR 5 drives and feels like a 'normal; car. It has a top speed of over 1 5 0 h d h (90mph), with a power output of75 kW (100bp). It is also believed that this vehicle wlll require less maintenance. It combines the low emission levels, the quietness and the smoothness associated with electric vehicles, while delivering a performance sinular to that ofan automobile with an internal combustion enginr. In April I Y99 the California Fuel Cell Partnership was developed. Foundmg members included DainderChrysler.the California An Resources Board, the Cahfornia Energy Commission, Ballard Power Systems, Ford, Shell and Texaco. The primary goals of the partnership are to:
Fig. 7 PEM fuel-cell distributed power plant. This unit, produced by Ballard Power Systems, provides 250kW heat and electricity which is enough power for a small building, a school or a community of up to 50 homes (photo: courtesy of Ballard Power Systems)
Demonstrate vehiclr technology by operating and testing the vehicles under real-world conditions in California. Demonstrate the viability of alternative fuel infrastructure technology, including hydrogen and methanol stations. Explorr the path to conunercialisation, from identifylng potential problcms to developing solutions. Increase public awareness and enhance opinion about fuel cell electric vehicles, preparing the market for commercialisation.
Fig. 8 Fuel-cell cogeneration power plant for residential applications, providing 7kW heat and electricity, enough power for a modern energy efficient home (photo: courtesy of Plug Power)
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Fig. 9 Prototype portable fuel cell providing 50W electrical power, produced by Heliocentris. The fuel cell is contained in the upper compartment; the hydrogen is stored in a metal hydride canister in the lower compartment
Since then new participants include General Motors, Honda, Hyundai, Nissan, Toyota, Volkswagen, British Petroleum, Exxon Mobil, US Department of Energy and US Department ofTransportation. To date five of the world's leading auto manufacturers have announced that they plan to introduce fuel cell automobiles beginning in the 2003 to 2005 timeframe. There are also plans {or buses and truck, all powered with fuel cell engines. In 2000, Ballard completed a two-year program of in-service field testing with six fuel cell buses, thrrc in Vancouver, British Colunibia, and three in Chicago. Objectives of the 6eld test included gathering data on performance and maintenance for me in Ballard's future heavy-duty engine designs, assessing public acceptance, and determining the needs of transit authorities and users of the buses. The results of the tests were exeniplary-the six buses travelled alnlost 75 000 miles and carried over 200 000 passengen. Thirty new transit buses powered by Ballard's heavy-duty fuel cell engines will be introduced to ten European cities beginning in 2003 for additional demonstration. The resulting data will be used to further develop a commercial fuel cell bus. Dbtribured power p x e r d t i o n Electrical energy demands throughout the world are continuing to increase. In Canada the demand is growing at an annual rate of approximately 2.6%. In America the rate is about 2.4%?, and in developing countries it is appmximatcly 696." How can these energy demands be niet responsibly and safely? Distributed power plants using fuel cells can provide part of the solution.
Distributed or 'decentrahsed' power plants, coutrastrd with centmlised power plants, are plants located close to the consumer, with the capability of providing both heat and electrical power (a combination known as 'cogeueration'). Heat, the byproduct ofelectrical power generation, is transferred from the fuel cell to a heat exchanger. The exchanger transfers the heat to a water supply, providing hot watcr to local customers. The overall efficiency of a cogeneration system can he iu excess of SOY,, comparatively high compared to a system producing electricity alone. An increase in efficiency naturally corresponds to a decrease in fuel consuniption. Distributed power plants have many additional advantages. For example, they can provide power to a remote location without the need of transporting electricity through transmission lines from a central plant. There is also an efficiency benefit in that the cost of transporting fuel is more than ofiet by the elinunation of the electrical losses of transmission. The ability to quickly build up a power infrastructure in developing nations is ohen cited. Using fuel cell power plants obviates the need for an electrical grid. Distributed power plants can provide either primary or back-up power. In primary applications they can provide base-load power, operating virtually continuously from the consumption of natural gas, reducing the demand from the electrical grid. This not only decreases the cost ofdisplaced power, but can also result in a reduction of demand charges imposed by the utility Should the power plant provide an excess of electricity, the excess can be fed back into the electrical grid, resulting in additional savings. In case of a power outagc on the grid, a distributed power plant can continue to pmvidt! power to cssentid services; eliminating the need for both an uninterruptible power supply (UPS),presently handled by lead-acid battery banks, and a standby generator, for exteiided period? of power outage. An additional quality of a fuel cell power plant for UPS applications is that the average 'down time' is anticipated to be low, 3.2 to 32 seconds per year against typically nine hours for a conventional battery-hank UPS (Internrt source: H D R Engineering). For industries where UPS systems are critical, such as banking, minimising dowii time is of utmost importance. Other applications for fuel cell dmributed power plants are also possible, e.g. stand-alone back-up power generators. The PEM fuel cell plant can be started in seconds, supplying power for as long as required from stored hydrogen, producing electrical power clearlly and virtually silently. Shown in Fig. 7 is a prototype l i d cell distributed power plant, by Ballard Power Systems. This unit pmvides 2SOkW of electricity and an cyuivalent amount ofheat. This is enough power for a community of about SO homes, or a small hospital or a remote school. This particular unit incorporates a fuel processor so that natural gas cau be used as a fiiel. The fuel processor convrro the natural gas, through the
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process of reformation, into a hydrogen-rich gas composed primarily of hydrogen and carbon dioxide. The hydrogen is used by the fuel cell and the carbon dioxide is released into the atmosphere. Eventually as an infrastructure for hydrogen develops, these units could be powered with hydrogen directly without the need for a fuel processor. Ballard Power is presently field-testing five of these units in the United States, Germany, Japan and Switzerland, with four more units planned for 2002. Testing is expected to continue until 2004.
compressed hydrogen
lithium-ionbattery
lead-acid battery
storage density by volume
300
r c
Residential power Fuel cell power plants are also > being developed by several g 100 c manufacturers to provide 50 electricity and heat to single-fady homes. Fuelled by either natural 0 compressed hydrogen lithium-ionbattery lead-acidbattery gas or propane, these plants will be able to supply base-load power or storage density by weight all the electricity required by a modern-day home. Fig. 10 Comparison of the energy density of compressed hydrogen (3000 psi) Ballard Power Systems, in against lithium-ion and lead-acid batteries collaboration with its associate purchase and install a larger number of units in a variety company Ebara Ballard, partner Ebara Corporation, and co-developer Tokyo Gas, has developed a 1kW of locations in the future. fuel cell generator designed to supply both base-load Portable power electrical power as well as heat to a dwelling. This unit can also be fuelled by natural gas. It does not provide Several manufacturers are also developing fuel cell enough power to supply the total electrical demands of power supplies for portable applications, providmg a a residence, but it does shift a portion of the demand few watts up to several kilowatts of electricity (Fig. 9). from the electrical grid to natural gas. The electrical Fuelled by stored natural gas, propane, methanol or efficiency of this fuel cell system is rated at 42% and the hydrogen gas, portable fuel cells may one day replace heat efficiency is rated at 43%. Therefore the combined both gasoline and desel-engine generatorsfor portable applications as well as conventional batteries for uses cogeneration efficiency of the system can be as h g h as such as remote lighting, laptop computers and mobile 85%. This particular generator is targeted at the Japanese residential market. Ballard’s goal is to phones. commence sales of these units in 2004. Compared with engine-driven mobile electrical generators, fuel cells have the significant advantage of Plug Power, based in Latham New York has developed a new fuel cell power plant that supplies being quiet and having low emissions. As they have few 5 kW of electricity plus heat, using natural gas as a fuel moving parts (only external pumps and fans) they (Fig. 8). Dependmg on the size and location of the operate virtually silently. If stored hydrogen is the fuel, house this could be enough power to supply the again the only emission is pure water. electrical demand of a modern energy efficient home. A significant advantage of the he1 cell over its battery Initially these fuel cell power plants will be operated in counterpart is that of its energy density (Fig. 10). parallel with the grid. Eventually they will be able to Portable power packs using fuel cells can be lighter and operate either grid parallel or grid independent, smaller in volume for an equivalent amount of energy, possibly supplying the entire power for a modern particularly the direct methanol &el cell. Note that the home. Plug Power is presently installing and testing comparison here is the he1 tank. these units in selected sites throughout North America, Europe and Japan. In 2001, 75 units were delivered to ‘ T h ef u e l cell makes sense when the energy storage required Long Island Power Authority (LIPA) to supplement the by an application represents many hours o operation at full f grid and provide learning experience for LIPA power. T h e durability o batteries i n this sort o application f f employees. This experience will enable LIPA to is at best a f e w hours. T h e size, weight, and cost o energy f
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discharge. Also the charge capacity of a rechargeable battery decreases with the number of times of charge and discharge. Conversely, provided that the hydrogen supply is sealed correctly, a fuel cell will not discharge over time, maintaining its full charge capacity almost indefinitely. Direct methanol fuel cells were invented and initially developed at theJet Propulsion Laboratory in Pasadena, California. They were designed to supply electricity for field troops in the Armed Forces and for applications with NASA (Fig. 11). The direct methanol, fuel cell has the advantage over the hydrogen fuel cell in that they can use a liquid fuel, i.e. methanol without the need for external reforming. Liquid Fuel is easy to store and has a high energy density compared to compressed hydrogen. At present, the direct methanol fuel cell suffers From relatively low efficiency and high cost, owing to required platinum loadmg compared to that ofthe hydrogen fuel cell. However, as this improves, it is expected that the direct methanol fuel cell will play a leading role in providing power for portable and possibly transportation applications. Ballad Power Systems, Motorola, the Los AIanios National Laboratory and Manhattan Scientific are all actively pursuing the development of the direct methanol fuel cell. Motorola claim that a portable cell phone wiU he able to remain fully charged on standby for a month rather than &ys. The company has also announced that it plans to have its version commercially available in three to five years.
I
Fig. 11 Prototype direct methanol fuel cell used as a lithium battely charger provides up to 20W electrical power (photo: courtesy of Jet Propulsion LaboratorieslNASA)
a fuel cell power plant easily out-competes batteries. You do have thefixed cost (and size and wekht) of the plant, which is afunction of power. 7his is why it is important to note that the advanrage offuel cells isfor low power, hkh eneryy applicationr.’ (Ric Pow of Pow Consulting, 2001)
storage f.r
Rechargeable batteries will discharge over time; the colder the ambient temperature the quicker they
The science of the PEM fuel cell
T h e chemistry o a single cell f In a PEM fuel cell, hvo half-cell reactions take place simultaneously,an oxidation reaction (loss ofelectrons) at the anode and a reduction reaction (gain ofelectrons) at the cathode. These two reactions make up the total oxidation-reduction (redox) reaction of the fuel cell, the formation of water from hydrogen and oxygen gases. As in an electrolyser, the anode and cathode are separated by an electrolyte, which allows ions to be transferred &om one side to the other (Fig. 12). The electmlyte in a PEM fuel cell is a solid acid supported within the membrane. The solid acid electrolyte is saturated with water so that thr transport of ions can proceed. The chemical reactions for a PEM fuel cell are:
anode reaction: &+2H+ + 2ecathode reaction: XO2 + 2e- + 2Hi+H?Og) overall reaction: H2 + X o + H 2 0 O2
(6)
(7)
(8)
Fig. 12 Schematic of a single PEM fuel cell. When an electrical load is attached across the anode and the cathode of the fuel cell a redox reaction occurs. The working voltage produced by one cell in this process is between 0.5 and O W , dependingon the load. To create practicalworking voltages, individualfuel cells are stacked together in series to form a fuel cell stack
At the anode, the hydrogen molecules first come into contact with a platinum catalyst on the electrode surface. The hydrogen molecules break apart, bonding to the platinum sudace forming weak H-Pt bonds. As the hydrogen molecule is now broken the oxidation reaction can proceed. Each hydrogen atom releases its
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electron, which travels around the external circuit to the cathode (it is this flow of electrons that is referred to as electrical current). The remaining hydrogen proton bonds with a water molecule on the membrane surface, forming a hydronium ion (H@+). The hydronium ion travels through the membrane material to the cathode, leaving the platinum catalyst site free for the next hydrogen molecule. At the cathode, oxygen molecules come into contact with a platinum catalyst on the electrode surface. The oxygen molecules break apart bondmg to the platinum surface forming weak 0-Pt bonds, enabling the reduction reaction to proceed. Each oxygen atom then leaves the platinum catalyst site, combining with two electrons (which have travelled through the external circuit) and two protons (whch have travelled through the membrane) to form one molecule of water. The redox reaction has now been completed. The platinum catalyst on the cathode electrode is again f?ee for the next oxygen molecule to arrive. Ths exothermic reaction, the formation of water from hydrogen and oxygen gases, has an enthalpy of -286kJ of energy per mole of water formed. The &ee energy avadable to perform work decreases as a function of temperature. At 25"C, one atmosphere, the free energy avadable to perform work is about -237kJ/mole. Ths energy is observed as electricity and heat.
polytetrafluoroethylene (PTFE) chains
F F F F F F F F F F F F F F F
-C-C-C-C-C-C-C-C-C-C-c-c-c-c-c-
I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I F F F F F F F O F F F F F F F
Fig. 13 Chemicalstructure of a PEM fuel cell membranelong chains of PTFE (Teflon) with side chain ending with sulphonic acid (HSOs) (source: Reference 2)
Polymer electrolyte (or proton exchange) membrane ('EM) The membrane material used in a PEM cell is a polymer. PEMs are generally produced in large sheets. The electrode catalyst layer is applied to both sides, and is cut to the appropriate size. A single PEM sheet is typically 50-175pm thck, or around the thckness of 2-7 sheets of paper. A common PEM material used today is Nafion. Developed in the 1970s by Dupont, Nafion consists of polytetrafluoroethylene (PTFE) chains, commonly known as Teflon, forming the backbone of the membrane. Attached to the Teflon chains, are side chains endmg with sulphonic acid (HS03) groups (Fig. 13).A close-up view of the membrane material shows long, spaghetti-like chain molecules with clusters of sulphonate side chains (Fig. 14). An interesting feature of this material is that, whereas the long chain molecules are hydrophobic (repel water), the sulphonate side chains are hghly hydrophylic (attract water). For the membrane to conduct ions efficiently the sulphonate side chains must absorb large quantities of water. Withn these hydrated regions, the hydrogen ions of the sulphonic acid groups can then move heely, enabling the membrane to transfer hydrogen ions, in the form of hydronium ions from one side of the membrane to the other.
Cell voltage and ejiciiency If the hel, cell was perfect at transferring chemical energy into electrical energy, the ideal cell voltage
(thermodynamic reversible cell potential) of the hydrogen he1 cell would be at 25"C, one atmosphere, 1.23V. As the he1 cell heats up to operating temperature, around 80°C the ideal cell voltage drops to about 1.18V. However, there are many limiting factors that reduce the &el cell voltage further. The voltage out of the cell is a good measure of electrical
Fig. 14 Close-up of a PEM fuel cell membrane. The Figure shows long spaghetti-like chain moleculesof Teflon surrounding clusters of hydrated regions around the sulphonate side chains. The Teflon chains form the backbone of the membrane. The hydrated regions around the sulphonate side chains become the electrolyte (source: Reference 2)
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'
Fig. 15 Graph comparing carbon dioxide emissions of cars, using different types of fuel sources (reprinted by permission from Pembina Institute). a Car with internal combustion engine; b Fuel cell car with hydrogen produced from Alberta electric grid (coal generation); c Fuel cell car with onboard gasoline reformer; d Fuel cell car with onboard methanol reformer; e Fuel cell car with hydrogen produced from natural gas (distributed from urban retail outlets); f Fuel cell car with hydrogen produced from natural gas (made at large refineries)
300 250
-
~
-
200 150 -
1
100
50
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0
efficiency; the lower the voltage, the lower the electrical efficiency and the more chemical energy is released in the formation of water and transferred into heat. The primary losses that contribute to a reduction in cell voltage are:
therefore important to remove this excess water, hence the term mass transport.
Direct methanol fuel cell
A direct methanol fuel cell also uses a PEM membrane. However, other catalysts in addtion to platinum are required on the anode side of the membrane to break the methanol bond in the reaction forming carbon dioxide, hydrogen ions and free electrons. As with the hydrogen fuel cell, the free electrons flow from the anode of the cell through an external circuit to the cathode and the hydrogen protons are transferred through the electrolyte nienibrane. At the cathode the Gee electrons and the hydrogen protons react with oxygen to form water. The chemical reactions of the direct methanol fuel cell are:
anode reaction: CH30H + H20 +COZ 6H' + 6e- (9) + cathode reaction: 31202 + 6Hf + 6e-+ 3H20 (10) overall reaction: CH30H + 3/202 -+ C02 + 2H20 (11)
Actiuation losses: Activation losses are a result of the energy required to initiate the reaction. This is a result of the catalyst. The better the catalyst the less activation energy is required. Platinum forms an excellent catalyst; however, there is much research underway for better materials. A limiting factor to power density available from a fuel cell is the speed at which the reactions can take place. The cathode reaction (the reduction of oxygen) is about 100 times slower than that of the reaction at the anode, thus it is the cathode reaction that limits power density. Fuel crossouer and internal currents: Fuel crossover and internal currents are a result of fuel that crosses directly through the electrolyte, from the anode to the cathode without releasing electrons through the external circuit, thereby decreasing the efficiency of the fuel cell. Ohmic losses: Ohmic losses are a result of the combined resistances of the various coniponents of the fuel cell. Ths includes the resistance of the electrode materials, the resistance of the electrolyte membrane and the resistance of the various interconnections. Concentration losses (also referred to as 'mass transport'): These losses result from the reduction of the concentration of hydrogen and oxygen gases at the electrode. For example, following the reaction new gases must be made immedately available at the catalyst sites. With the build up of water at the cathode, particularly at high currents, catalyst sites can become clogged, restricting oxygen access. It is
Where will the hydrogen come from?
One of the most important questions to be asked is: where will the hydrogen come from?A very interesting study published by the Pembina Institute, based in Calgary, Alberta, compared total carbon dioxide emissions of fuel cell vehicles using hydrogen produced from a variety ofmethods (Fig. 15). The results clearly show that the choice as to which method will be used to produce the hydrogen will be a critical environmental decision. For example, if hydrogen is produced from the electrolysis of water and the electrolysers are powered from the electrical grid, whereby the electricity is produced from a coal burning power station, then there will be no reduction in carbon dioxide emissions compared with the levels of the present day internal
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combustion engine. In fact, there will be an increase in metals and pollutants into the environment. If on the other hand the electrolyser is powered from a renewable energy source, through use of a solar panel, a wind turbine or a hydroelectric turbine, there will be no emissions of carbon dioxide. As the fuel cell industry grows there will likely be different approaches to niahng hydrogen in various areas of the world, depending on their local energy production methods.
solar cell
wind
micro hydro
Reformation of hydrocarbon fucls For the short term, because of the abundance of natural gas, the availability of Fig. 16 Electrical power from renewable energy sources. In the past, the limiting factors methanol and propane, and of renewable energy have been the storage and transport of that energy. With the use of the lack of a hydrogen an electrolyser, a method of storing and transporting hydrogengas, and a fuel cell, infrastructure, it is expected electricalpower from renewable energy sources can be deliveredwhere and when that hydrocarbon fuels will required, cleanly, efficiently and sustainably be the dominant fuels for stationary fuel cell applications. For as long as these These systems are truly sustainable, for as long as fuels are cheaply available, reformation of a there is sunlight there can be electrical power, available hydrocarbon fuel is the most cost efficient method of where and when required. producing hydrogen. In the reformation of a hydrocarbon fuel, however, there is an emission of Biological methods carbon &oxide. Although carbon dioxide is not Research and development is talung place on the considered a pollutant, controversy exists that manproduction of hydrogen from biological methods made emissions niay contribute to global warming. (biohydrogen). For example, Dr. A. Melis at the University of California, Berkeley, has discovered a Renewable energy systems metabolic switch in common green algae Hydrogen can be produced sustainably with no (chlamydomonas reinhardtill that causes the algae to emission of carbon &oxide from renewable energy oxidise water and to produce pure hydrogen gas when systems. An example of such a system is the use of a sulphur nutrients are depleted froin the growth solar panel, a wind turbine or a micro-hydro generator medium. This and other biohydrogen mechanisms are to convert the ra&ant energy of sunhght into electrical presently in the R&D stage but may one day provide power, which drives an electrolyser. The electrolyser the world with an additional source of hydrogen. breaks apart water producing hydrogen and oxygen gases. The hydrogen is stored for use by the fuel cell and Benefits and obstacles the oxygen is released into the atmosphere. Thus when the sun shines, the wind blows or the water flows, the Ben$ts electrolyser can produce hydrogen. Fuel cells are e$icient: They convert hydrogen and A power system incorporating hydrogen from oxygen directly into electricity, water and heat, with renewable sources and a fuel cell is a closed system, as no combustion in the process. The resulting none of the products or reactants, water, hydrogen and efficiency is between 50 and 80%, about double that oxygen, is lost to the outside environment. The water of an internal combustion engine. consumed by the electrolyser is converted to gases. The Fuel (ells are clean: If hydrogen i the fuel, there are no s gases are converted back to water. The electrical energy pollutant emissions &om a fuel cell itseE only the produced by the solar panel is transferred to cheinical n production of pure water. In contrast to a internal energy in the form ofgases. The gases can be stored and combustion engine, a bel cell produces no emissions of transported, to be reconverted back to electricity sulphur dioxide, which can lead to acid rain, nor nitrogen (Fig. 16). oxides whch produce smog nor dust particulates.
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Fuel cells are quiet: A b e l cell itself has no moving parts, although a fuel cell system may have pumps and fans. As a result, electrical power is produced relatively silently. Many hotels and resorts in quiet locations, for example, could replace dese1 engine generatorswith &el cells for both main power supply or for backup power in the event of power outages. Fuel cells are modular: That is, fuel cells of varying sizes can be stacked together to meet a required power demand. As mentioned earlier, fuel cell systems can provide power over a large range, &om a few watts to megawatts. Fuel cells are environmentally safe: They produce no hazardous waste products, and their only byproduct is water (or water and carbon dioxide in the case of methanol cells) and heat. Fuel cells may give us the opportunity to provide the world with sustainable electrical power. Obstacles At present there are many uncertainties to the success of fuel cells and the development of a hydrogen economy: Fuel cells must obtain mass-market acceptance to succeed: Thls acceptance depends largely on price, reliabhty, longevity of fuel cells and the accessibhty and cost of fuel. Compared to the price of present day alternatives e.g. desel-engine generators and batteries, fuel cells are comparatively expensive. In order to be competitive, fuel cells need to be massproduced and less expensive materials developed. An infvastructure for the mass-market availability of hydrogen, or methanolfuel initially, must also develop: At present there is no infi-astructure in place for either of these fuels. As it is we must rely on the
activities of the oil and gas companies to introduce
Conclusion
As our demand for electrical power grows, it becomes increasinglyurgent to find new ways ofmeeting it both responsibly and safely. In the past, the limiting factors of renewable energy have been the storage and transport of that energy. With the use of fuel cells and hydrogen technology, electrical power from renewable energy sources can be delivered where and when required, cleanly, efficiently and sustainably.
Acknowledgments
The author would &e to thank Rachel Browne for supplying the graphs and drawings and editing the text, and Ric Pow, h m Pow Consulting,Vancouver, Canada, for reviewing the paper and providmg t e c h c a l advice.
References
1 CONNIHAN, M. A,: ‘Dictionary of energy’ (Routledge and Kegan Paul, 1981) 2 LARMINIE, J., and DICKS, A,: ‘Fuel cell systems explained’ (John Wiley & Sons, 2000) 3 BERRY, M., and MACDONALD A.: ‘Energy through hydrogen’ Heliocentris 4 KOPPEL, T.: ‘Poweringthe future’(JohnWiley and Sons, 1999) 5 INTERNATIONAL ENERGY AGENCY ‘Energy policies of IEA countries’ (OECD Publications, 1997) 6 KHATIB,H.: ‘Electricalpower in developing countries’,Power EngineeringJournal, October 1998, 12, (lo), pp. 239-247 7 COLELL, H.: ‘Solar hydrogen technology’ (Heliocentris, 1998) 8 MELIS, A,, and HAPPE, T.: ‘Hydrogen production:green algae as a source of energy’, Plant Physiology, 2001, 127:pp. 740-748 9 THOMAS, S., and ZALBOWITZ, M.: ‘Fuel cells, green power’ &os Alamos Nanonal Laboratory, 1999, booklet)
internet sources
Power System: http://www.ballard.com/25Okstationary.asp HDR Engineering and Architecture: hap://www.hdrinc.com/ information/search.asp?PageID=476 IFC: http://www.in~rnationalfuelcells.com/spacedefe~e/ heritage. shtml Jet Propulsion Laboratory: http://www2.jpl.nasa.gov/til~/unages/ captiom/p486OO.txt Los Alamos National Laboratory: http://www.lanl.gov/worldview/ science/features/helcell. html NASA: Gemini: http://scienCe.ksc.nasa.gov/history/gemigemjnv/Fmini-v.htd Space Shuttle Orbiter: http://science.ksc.nasa.gov/shuttle/ technology/sts-newsref/sts-eps. html Pembina Institute: http://www.pembina.org/pubs/pa ibelcell.pdf Plug Power: http://www.plugpower.com/technology/ Smithsonian Institution: http://americanhistory.si.edu/csr/fuelcells/ pem/pemmain.hm
them. Unless motorists are able to obtain fuel conveniently and affordably, a mass market for motive applications will not develop. A t present a large portion of the investment infuel cells and hydrogen technology has come from auto manujicturers: However, if fuel cells prove unsuitable for automobiles, new sources of investment for &el cells and the hydrogen industry wdl be needed. Changes in government policy could also derail fuel cell and hydrogen technology development: At present stringent environmental laws and regulations, such as the Cahfornia Low Emission Vehcle Program, have been a great encouragement to these fields. Deregulation laws in the uthty industry have been a large impetus for the development of distributed stationary power generators. Should these laws change it could create
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supplies to the world platinum market are &om South Africa, Russia and Canada. Shortages of platinum are not anticipated; however, changes in government policies could af€ect the supply.
cells and hydrogen’ technolib equipment for schools and universities. Mr Cook has a background in science kom University of British Columbia and electrical technology, and has a special interest in environmental sustainability. He can be contacted at: [email protected].
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